Linköping Studies in Science and Technology Dissertation No: 931 Studies of MISiC-FET sensors for car exhaust gas monitoring Helena Wingbrant Linköping 2005 S-SENCE and Division of Applied Physics Department of Physics and Measurement Technology Linköping University c 2005 Helena Wingbrant ISBN 91-85297-61-5 ISSN 0345-7524 Printed in Sweden by UniTryck, Linköping 2005 Abstract The increasing size of the car fleet makes it important to find ways of lowering the amounts of pollutants from each individual diesel or gasoline engine to almost zero levels. The pollutants from these engines predominantly originate from emissions at cold start, in the case when gasoline is utilized, and high NOx emissions and particulates from diesel engines. The cold start emissions from gasoline vehicles are primarily due to a high light-off time for the catalytic converter. Another reason is the inability to quickly heat the sensor used for controlling the air-to-fuel ratio in the exhausts, also called the lambda value, which is required to be in a particular range for the catalytic converter to work properly. This problem may be solved utilizing another, more robust sensor for this purpose. One way of treating the high NOx levels from diesel engines is to intro- duce ammonia in the exhausts and let it react with the NOx in a special catalytic converter to form nitrogen gas and water, which is called SCR (se- lective catalytic reduction). However, in order to make this system reduce NOx efficiently enough for meeting future legislations, closed loop control is required. To realize this type of system an NOx or ammonia sensor is needed. This thesis presents the efforts made to test the SiC-based field effect sensor device both as a cold start lambda sensor for gasoline engines and as an NH3 sensor for SCR systems in diesel engines. The MISiC (metal insulator silicon carbide) lambda sensor has proven to be both sensitive and selective to lambda, and its properties have been stud- ied in lambda stairs both in gasoline engine exhausts and in the laboratory. There is, however, a small cross-sensitivity to CO. The influence of metal gate restructuring on the linearity of the sensor has also been investigated. The metal tends to form islands by time, which decreases the catalytic activity and thereby gives the sensor, which is binary when fresh, a linear behavior. Successful attempts to prevent the restructuring through depositing a pro- tective layer of insulator on top of the metal were made. The influence of increasing the catalytic activity in the measurement cell was also studied. It iii iv Abstract was concluded that the location of the binary switch point of MISiC lambda sensors could be moved towards the stoichiometric value if the consumption of gases in the measurement cell was increased. The MISiC NH3 sensor for SCR systems has been shown to be highly sensitive to ammonia both in laboratory and diesel engine measurements. The influence of other diesel exhaust gas components, such as NOx, water or N2O has been found to be low. In order to make the ammonia sensor more long-term stable experiments on samples with different types of co-sputtered Pt or Ir/SiO2 gas-sensitive layers were performed. These samples turned out to be sensitive to NH3 even though they were dense and NH3 detection normally requires porous films. The speed of response for both sensor types has been found to be fast enough for closed loop control in each application. Contents List of papers vii Acknowledgements xi 1 Introduction 1 1.1Alternativeenergysources.................... 2 1.2Exhaustgasafter-treatment................... 4 1.2.1 Gasolineengineexhausts................. 4 1.2.2 Lean-burnengineexhausts................ 6 1.3Thescopeofthisthesis...................... 8 2 Cold start lambda sensor 9 2.1Sensorrequirements........................ 11 2.2Themetaloxidesensor...................... 11 2.2.1 Sensingprinciple..................... 11 2.2.2 Thebinaryzirconiasensor................ 14 2.2.3 Thelinearzirconiasensor................ 15 2.2.4 The TiO2 sensor and the SrTiO3 sensor......... 16 2.3Thefieldeffectsensor....................... 16 2.3.1 Sensingprinciple..................... 17 2.3.2 TheSiCcoldstartsensor................. 19 3 NH3 sensor for SCR systems 23 3.1Sensorrequirements........................ 24 3.2Thezeolite-basedsensor..................... 24 3.3Themixed-potentialzirconiasensor............... 26 3.4Themolybdenumoxidesensor.................. 27 3.5TheSiCfieldeffectsensor.................... 27 3.5.1 Sensingprinciple..................... 27 3.5.2 The SiC NH3 sensor................... 28 v vi Contents 4 The SiC field effect device 29 4.1ThepropertiesofSiC....................... 29 4.2Thedifferenttypesoffieldeffectsensors............ 31 4.2.1 TheSiCcapacitor..................... 32 4.2.2 TheSiCSchottkydiode................. 35 4.2.3 TheSiCtransistor.................... 38 5 The sensitive layer 43 5.1Sputterdeposition........................ 44 5.2Evaporation............................ 46 5.3 Long-term stability ........................ 47 5.4 Influence of catalytic activity .................. 48 6 Methods 51 6.1Productionandmounting.................... 51 6.2Lambdasensorforclosedloopcontrol.............. 54 6.3AmmoniasensorforSCRsystems................ 57 7Results 59 7.1Lambdasensorforclosedloopcontrol.............. 59 7.2AmmoniasensorforSCRsystems................ 61 8 Future work 63 References 65 List of papers This thesis is based on the following papers: I. H. Wingbrant, H. Svenningstorp, P. Salomonsson, P. Tengström, I. Lund- ström and A. Lloyd Spetz, “Using a MISiC-FET device as a cold start sensor”, Sensors and Actuators B, 93 (2003) 295-303. II. H. Wingbrant, I. Lundström and A. Lloyd Spetz, “The speed of re- sponse of MISiC-FET devices”, Sensors and Actuators B, 93 (2003) 286-294. III. H. Wingbrant and A. Lloyd Spetz, “Dependence of Pt gate restructur- ing on the linearity of SiC field effect transistor lambda sensors”, Sensor Letters, 1 (2003) 37-41. IV. H. Wingbrant, H. Svenningstorp, P. Salomonsson, D. Kubinski, J.H. Vis- ser, M. Löfdahl and A. Lloyd Spetz, “Using a MISiC-FET sensor for detecting NH3 in SCR systems”, IEEE Sensors Journal, accepted. V. H. Wingbrant and A. Lloyd Spetz, “The influence of catalytic activity on the binary switch point of MISiC-FET lambda sensors”, Applied Catalysis, submitted. VI. H. Wingbrant, M. Persson, A.E. Åbom, M. Eriksson, B. Andersson, S. Simko, D. Kubinski, J.H. Visser and A. Lloyd Spetz, “Co-sputtered metal and SiO2 layers for use in thick-film MISiC NH3 sensors”, IEEE Sensors Journal, submitted. VII. H. Wingbrant, M. Lundén and A. Lloyd Spetz, “Modifications of the catalytic gate material to increase the long-term stability of SiC-FET lambda sensors”, in manuscript. vii viii List of papers Related papers, not included in this thesis: VIII. O. Weidemann, M. Hermann, G. Steinhoff, H. Wingbrant, A. Lloyd Spetz, M. Stutzmann and M. Eickhoff, “Influence of surface oxides on hydrogen-sensitive Pd:GaN Schottky diodes”, Applied Physics Letters, 83 (2003) 773-775. IX. A. Lloyd Spetz, L. Unéus, H. Svenningstorp, H. Wingbrant, C. Harris, P. Salomonsson, P. Tengström, P. Mårtensson, P. Ljung, M. Mattsson, J.H. Visser, S.G. Ejakov, D. Kubinski, L.-G. Ekedahl, I. Lundström and S. Savage, “MISiCFET chemical gas sensors for high temperature and corrosive environment applications”, Material Science Forum, 389-393 (2002) 1415-1418. X. H. Wingbrant, L. Unéus, M. Andersson, J. Cerdà, S. Savage, H. Sven- ningstorp, P. Salomonsson, P. Ljung, M. Mattsson, J.H. Visser, D. Ku- binski, R. Soltis, S.G. Ejakov, D. Moldin, M. Löfdahl, M. Einehag, M. Persson and A. Lloyd Spetz, “MISiCFET chemical sensors for ap- plications in exhaust gases and flue gases”, Material Science Forum, 433-436 (2003) 953-956. XI. S. Nakagomi, M. Takahashi, L. Unéus, S. Savage, H. Wingbrant, M. An- dersson, I. Lundström, M. Löfdahl and A. Lloyd Spetz, “MISiC-FET devices with bias controlled baseline and gas response”, Conference pro- ceedings IEEE sensors 2003, Toronto, Canada, 22-24 October (2003). XII. D. Briand, H. Wingbrant, H. Sundgren, B. van der Schoot, L.-G. Eke- dahl, I. Lundström and N.F. de Rooij, “Modulated operating tempera- ture for MOSFET gas sensors: hydrogen recovery time reduction and gas discrimination”, Sensors and Actuators B, 93 (2003) 276-285. XIII. I. Belov, H. Wingbrant, A. Lloyd Spetz, H. Sundgren, B. Thuner, H. Svenningstorp and P. Leisner, “Thermal and flow analysis of SiC- based gas sensors for automotive applications”, Proc. of the 5th IEEE conference EuroSimE 2004, Brussels, May 9-12 (2004) 475-482. List of papers ix My contribution to the papers included in the thesis: Paper I All experimental work, data evaluation and writing Paper II All experimental work, data evaluation and writing Paper III All experimental work, data evaluation and writing Paper IV The N2O measurements and SEM analysis, data evaluation and writing Paper V All experimental work, data evaluation and writing Paper VI Supervision of the experimental work, some of the Auger and SEM analyses, some of the data evaluation, all the writing Paper VII Supervision of the experimental work on capacitors, the transistor measurements, the SEM analyses, the data evalu- ation of the transistor measurements as well as the capaci- tor measurements to some extent, all the writing x Acknowledgements A trait of mine, that most people may not know so much about, is that what I read first when I have got a new thesis in my hands is. the acknow- ledgements. Sometimes, I must confess, it may even be the only